The inverters presently available on the market can be divided into two types according to their output waveforms: one of non-pure sine wave inverters, and the other of pure sine wave inverters. The non-pure sine wave inverters include square wave inverters, quasi sine wave inverters and etc.. The non-pure sine wave inverters can only apply to a resistive load, and will cause damage to the load and the inverter itself if it applies to an inductive load, a capacitive load, a rectification and filtering load. Therefore, this type of inverters only exist on the low-side market, and cannot be used in most instances.
The pure sine wave inverters are suitable for various types of loads. Moreover, if the amplitude and phase of its output signal is controlled to some extent, the output signal can be transmitted to a power grid (for example, a commercial power grid) so as to achieve a grid-connection. The pure sine wave inversion can be divided into three types according to their inverting solutions. The first type comprises a power frequency transformer solution, which is the earliest inverting solution and which requires a massive transformer, having a large volume, a heavy weight, a high cost and a low efficiency. The second type comprises a high frequency transformer solution, which only requires a smaller high frequency transformer, having improvement on volume, weight and efficiency, but no substantial improvement on cost because of its complex electric circuit.
In order to reduce the cost of the solar electric power generation system so as to make it popular, it is relatively desired to use a transformerless solution, which requires no transformer and is highly improved on volume, weight and cost. Moreover, in the solar electric power generation system, the cost of the solar cell is quite high, thus the efficiency of the inverter should be further increased. In addition, if the solar electric power generation system is grid-connected, many devices such as storage batteries can be omitted, so as to further reduce the cost of the system. If so, it is necessary for the amplitude and phase of the output signal of the electric power generation system to be controlled so as to be consistent with the inherent alternating current signal in the power grid. Since the transformerless solution does not achieve isolation between the electric power generation system and the power grid, safety of the transformerless solution is quite important for the transformerless grid-connection.
Therefore, it is a common desire in the art to design a transformerless, high-efficiency, high-safety solar grid-connecting inverting device.